With touch-screen and touch-display devices (collectively referred to herein as “touch-screen devices”) rapidly growing in popularity, one significant shortcoming over traditional methods of data-entry has quickly become evident. The lack of tactile sensations perceived by the user when pressing “virtual” buttons on the touch-screen—the feelings of button edges and depressing the button for example—necessitates extra concentration by the user, who must visually confirm the position of their finger(s) relative to the display and then check they have correctly entered the data after each action. Real buttons and keys help divide the mental effort amongst the senses with the sense of touch helping to limit the workload on the visual sensory system. It has been shown that data entry using virtual buttons, as opposed to traditional physical buttons, causes an increase in data entry error rates and a decrease in user satisfaction due to the lack of such realistic tactile sensations.
In conventional touch-screen devices, such touch-screen devices may be enhanced through the addition of a means to artificially create tactile sensations, a feature known as tactile feedback. For example, when the user touches the touch-screen in a location corresponding to that of a virtual button, a tactile feedback device may stimulate the user's finger to artificially re-create the sensation of touching a physical button.
An exemplary method to reproduce tactile sensations is to stimulate one's sense of touch through vibrations, or oscillatory motions, of the surface of the device in contact with the user's finger. The generated vibrations may be in a direction normal to the plane of the surface (herein normal motion) or in a direction along the plane of the touch-screen (herein lateral motion). Since the skin is essentially insensitive to the direction of the vibrating motion, either direction of motion is effective in reproducing tactile sensations.
Given that a touch-screen surface on a mobile device is typically formed by a single continuous piece of material—for example, glass or plastic—the sensations caused by mechanical vibrations are limited to the generation of a single tactile sensation across the entire surface. That is, the entire screen vibrates at once thus sending the same tactile signal to all the user's fingers regardless of their position on the device and regardless of whether providing feedback to all fingers is appropriate or not.
Another method of applying tactile stimuli to a touch-screen is with a technique known as electro-vibration. A tactile feedback device employing this technique typically includes a conductive layer just below its surface (insulated from the user) to which a periodic voltage signal of a large magnitude is applied. The user's skin is then periodically attracted toward the display by electrostatic forces which can be perceived by the user as tactile sensations. This principal of electro-vibration method was described in Mallinckrodt et al, SCIENCE 118(3062) 277, dated Sep. 4, 1953, and application of this method to tactile feedback devices is described in MÄKINEN, V. et al., WO2009141502, dated Nov. 26, 2009, and POUPYREV, I. et al., US20100085169, dated Apr. 8, 2010. Furthermore, it is known that by dividing up the conductive layer into separately addressable electrode segments, each electrode segment may be independently controlled to create a touch feedback device that permits independent tactile feedback sensations to be generated at specific and multiple locations. However, the use of electrode segments suffers from the disadvantage that it is necessary to independently address all segments. As a result, a large number of connections is required, and a significant proportion of the device surface area must be devoted to the wiring used for addressing. The electrode segment method is therefore not suitable for creating a tactile feedback device with a high spatial resolution.
A tactile feedback device based on a form of electro-vibration is also described in RADIVOJEVIC, Z. et al., US20100152794, dated Jun. 17, 2010. The device described therein may be addressed in such a manner as to permit independent tactile feedback sensations to be generated at specific and multiple locations across the device. To achieve this effect, rows and columns have certain positive or negative voltages applied to them. Only above the electrodes which mark the intersection of a row carrying a positive voltage and a column carrying a negative voltage (or vice versa), is a tactile sensation felt by the user. At other locations, only the positive or negative voltage signals exist and nothing is felt by the user. Such a method relies on the existence of a voltage threshold below which the human skin cannot sense the tactile signal. However, this voltage threshold may vary between users and depend on environmental conditions, thus leading to unreliable performance of the device.